Thermal sensors have long been used to measure power, particularly at RF and microwave frequencies. One type of conventional thermal sensor uses a pair of power-dissipating elements, such as resistors, with a related pair of temperature-sensitive elements thermally coupled to the power-dissipating elements. The input signal to be measured is applied to the first resistor, thereby causing it to couple thermal energy to the first temperature-sensitive element in accordance with the power of the input signal. The temperature-sensitive elements may be such semiconductor devices as junction diodes or bipolar transistors.
The signals from the first and second temperature-sensitive elements are each fed to an input of a differential amplifier which produces a DC output signal. This DC signal is fed to the second resistor and is caused to dissipate a power that equals the power dissipated by the input signal in the first resistor. When balance is achieved, the DC voltage developed across the second resistor by the differential amplifier is equivalent to the RMS or effective value of the input signal which causes the temperature rise in the first resistor. One implementation of this technique is shown in U.S. Pat. No. 3,668,428, issued to Henry Koerner. Attention is also directed to U.S. Pat. No. 4,023,099, issued to van Kessel, and to U.S. Pat. No. 4,257,061, issued to Chapel.
While this arrangement can have good sensitivity and works well at full-scale or high levels, it suffers several significant limitations inherent in its design. As an example, at low signal or power levels, there are several error effects that limit performance and reduce the useful range of the device. At very low input signal levels, noise in the temperature-sensitive elements is amplified by the necessarily large forward gain of the differential amplifier. The output noise from the differential amplifier is dissipated in the second resistor. The amplified noise power is an offsetting signal that degrades the linearity of the power sensor for low input signal levels. The noise also establishes a floor which the input signal must exceed in order to be detected. The noise thus limits the dynamic range of the power sensor. Other problems with these prior art thermal power sensors also exist, as described in greater detail below.